U.S. patent number 5,497,824 [Application Number 08/288,558] was granted by the patent office on 1996-03-12 for method of improved heat transfer.
Invention is credited to Mohammad A. Rouf.
United States Patent |
5,497,824 |
Rouf |
March 12, 1996 |
Method of improved heat transfer
Abstract
An improved method of heat transfer between a first
substantially homogenous fluid passing through the inside of a heat
exchange tube, and a second fluid on the outside of the heat
exchange tube includes the step of passing the first fluid through
a compound turbulator inside the tube. The compound turbulator
includes a twisted strip surrounded by a helical coil that is
closely spaced from the walls of the tube. Passing the first fluid
over the compound turbulator generates a high level of turbulence
in the first fluid in the region adjacent the tube wall,
facilitating heat transfer to the second fluid on the outside of
the heat exchange tube. The augmentation of heat transfer to the
second fluid can also be accomplished through the use of a helical
coil installed on the outside of the tube.
Inventors: |
Rouf; Mohammad A. (Glen Carbon,
IL) |
Family
ID: |
23853385 |
Appl.
No.: |
08/288,558 |
Filed: |
August 10, 1994 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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466861 |
Jan 18, 1990 |
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Current U.S.
Class: |
165/184; 138/38;
165/109.1 |
Current CPC
Class: |
F28F
13/12 (20130101) |
Current International
Class: |
F28F
13/00 (20060101); F28F 13/12 (20060101); F28F
013/02 () |
Field of
Search: |
;165/109.1,184,1
;138/38 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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984156 |
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Feb 1965 |
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GB |
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664018 |
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May 1979 |
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SU |
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705239 |
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Jan 1980 |
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SU |
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1453147A |
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Jan 1989 |
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SU |
|
Primary Examiner: Flanigan; Allen J.
Attorney, Agent or Firm: Heller & Kepler
Parent Case Text
This application is a continuation of application Ser. No. 466,861,
filed Jan. 18, 1990, now abandoned.
Claims
What is claimed is:
1. An improved method of heat transfer between a first fluid
passing through the inside of a heat exchange tube, and a second
fluid on the outside of the heat exchange tube, the improvement
comprising: passing the first fluid through a compound turbulator
inside the tube, the compound turbulator comprising a twisted strip
surrounded by a helical coil that is either in contact with or
closely spaced from the walls of the tube, to generate a high level
of turbulence in the first fluid in the region adjacent the tube
wall to facilitate heat transfer to the second fluid on the outside
of the heat exchange tube.
2. An improved method of heat transfer according to claim 1 wherein
this is a closely fitted helical coil on the outside of the heat
exchange tube, and further comprising the step of forcing the
second fluid to flow longitudinally along the outside of the tube
over the helical coil, to generate a high degree of turbulence in
the second fluid adjacent the outside surface of the tube.
3. An improved method of heat transfer between a first
substantially homogenous fluid passing through the inside of a heat
exchange tube, and a second fluid on the outside of the heat
exchange tube, the improvement comprising: passing the first
substantially homogeneous fluid through a compound turbulator
inside the tube, the compound turbulator comprising a twisted strip
surrounded by a helical coil that is either in contact with or
closely spaced from the walls of the tube, to generate a high level
of turbulence in the first substantially homogeneous fluid in the
region adjacent the tube wall to facilitate heat transfer to the
second fluid on the outside of the heat exchange tube.
4. An improved method of heat transfer according to claim 3 wherein
this is a closely fitted helical coil on the outside of the heat
exchange tube, and further comprising the step of forcing the
second fluid to flow longitudinally along the outside of the tube
over the helical coil, to generate a high degree of turbulence in
the second fluid adjacent the outside surface of the tube.
5. A compound turbulator for heat transfer enhancement for use in
combination with a heat exchanger tube, the compound turbulator
comprising:
a twisted strip within a helical coil,
wherein the compound turbulator is installed coaxially inside a
tube of a heat exchanger, and the helical coil being either in
contact with or closely spaced from an inner tube wall surface.
6. A compound turbulator described in claim 5 in combination with a
helical coil turbulator,
wherein the helical coil turbulator installed on or closely spaced
from an outer surface of the heat exchanger tube enhances the heat
transfer of the heat exchanger tube.
7. A compound turbulator described in claim 5 wherein the helical
coil of the compound turbulator comprises a coiled ribbon.
8. A compound turbulator described in claim 5 wherein the helical
coil of the compound turbulator comprises a coiled wire.
9. A compound turbulator described in claim 6 wherein the outer
helical coil turbulator spring comprises a coiled ribbon.
10. A compound turbulator described in claim 6 wherein the outer
helical coil turbulator comprises a coiled wire.
Description
BACKGROUND OF THE INVENTION
This invention relates to a improved method of heat exchange
between a first fluid inside a heat exchange tube and a second
fluid on the outside of the heat exchange tube.
When heat is transferred from one fluid to another fluid through a
solid wall (e.g., the wall of the tube) the magnitude of heat
transfer depends on (a) surface area, (b) the value of heat
transfer coefficient at the inside and outside surface of the tube,
(c) the thermal conductivity of the tube wall material, (d) the
wall thickness, and (e) temperatures of the participating fluid.
Therefore, for given fluids and their temperatures and for a given
tube material and size, the magnitude of heat transfer can be
significantly increased by augmenting heat transfer at the inside
or outside or on both sides of the tube wall. Such augmentation is
defined as the improvement of the convective heat transfer
coefficient. Therefore, the method of this invention is useful in
economizing design and manufacture of a large variety of heat
exchange equipment by reducing the heat transfer surface for given
magnitude of heat transfer. However the method of this invention is
simple enough to implement in new or existing heat transfer
equipment and economical enough to justify its cost compared with
the benefits it can derive.
A turbulator can most effectively improve heat transfer if it is
capable of generating a high level of turbulence in the boundary
layer very close to the wall surface. The turbulators that are
presently available in the market do not effectively generate such
high levels of turbulence close to the wall surface through which
heat transfer actually takes place. For example, the turbulators
disclosed in Smick, U.S. Pat. No. 4,044,796, and Burke, U.S. Pat.
No. 4,412,558, are both made of a metal strip of suitable width
formed in a zigzag shape. When such turbulators are placed inside a
tube, a portion of the tube opening is blocked and, therefore, the
fluid has to flow around the turbulator strip. As the fluid passes
over the strip, turbulence is generated primarily in the bulk fluid
due to flow separation. The turbulence thus created eventually
transmits into the boundary layer where the viscosity of the fluid
dampens down the intensity of the turbulence, meaning reduced
effectiveness in the augmentation of heat transfer. Moreover, the
zigzag strip is limited for the tube's internal augmentation and it
is not suitable for liquids, especially for viscous liquids due to
the high pressure drop and reduced effectiveness.
Another kind of turbulator presently available in the market is
known as a spinner turbulator, comprises a twisted metal strip. It
is inserted inside a tube. It is used for both liquid and gas. As
the flow progresses through the tube, the fluid gains spinning
motion as it follows the contoured path of the twisted strip and
thereby exerts centrifugal force on the tube wall and partially
distorts the boundary layer. Also the combined effect of spinning
and translatory motion of the fluid increases the turbulence level
in the bulk fluid which eventually transmits into the boundary
layer, but at a reduced intensity due to viscous effect of the
fluid. Again, this type of turbulator is limited for insertion
inside a tube.
The boundary layer turbulators used in the present invention are
effective heat transfer augmenters Their primary function, unlike
the other turbulators, is to generate a very high level of
turbulence right in the boundary layer very close to the wall
surface where heat transfer actually takes place. The turbulence
thus generated propagates from the boundary layer to bulk
fluid.
The comprised turbulator consists of a wire or ribbon coiled in the
form of a helical spring placed inside the tube to extend the
entire length of the tube. The outside diameter of the coil is
equal to, or slightly less than, the inside diameter of the tube
(in the case of a round tube) so that the wire or ribbon is placed
against the wall surface or very close to it. When the flow of
fluid trips over the wire, a high level of turbulence is generated
in the boundary layer close to the wall surface, thus augmenting
the heat transfer. Similarly, a helical coil can be put on the
outside surface of the tube for augmentation of heat transfer at
the outer surface when a fluid flows over the tube in the direction
of the tube axis. Such helical coil-like turbulator is called a
boundary layer turbulator. According to the method of this
invention, the heat transfer coefficient inside the tube can be
further increased when the boundary layer turbulator used inside
the tube in conjunction with a spinner turbulator. Under this
situation, centrifugal force due to circulatory motion of the bulk
fluid helps to generate even higher levels of turbulence in the
boundary layer extremely close to the wall and thus resulting in
even higher levels of heat transfer augmentation. The application
of such augmentation is only limited for the inside surface of the
tube.
The ultimate object of the method of this invention is to apply a
compound turbulator in an improved method of heat transfer in which
the heat transfer surface of the heat exchange equipment can be
significantly reduced. Thus, a significant savings can be obtained
in fabricating this equipment. Such heat exchange equipment can
exchange heat between any two fluids such as between a liquid and a
liquid, or a gas and a liquid. The first fluid inside the heat
exchange tube is highly turbulated, and thus substantially
homogenous.
SUMMARY OF INVENTION
According to the method of transferring heat between a first fluid
passing through the inside of a heat exchanger tube and a second
fluid on the outside of the tube of the invention, the first fluid
is passed over a compound turbulator disposed in the tube. The
compound turbulator comprises a first boundary layer turbulator
surrounding a spinner turbulator. The compound turbulator is a
turbulence generator in the boundary layer very close to the tube
wall surface. Such turbulence is generated by placing wire or
ribbon (herein referred to as wire) on the wall surface or closely
adjacent to the two wall surface in a direction perpendicular to
the direction of the flow. When the flow of fluid trips over the
wire, a very high level of turbulence is generated immediately
downstream of the wire due to the flow separation that takes place
as the fluid trips over the wire. Such boundary layer turbulence is
primarily responsible for heat transfer augmentation at the wall
surface. Such heat transfer augmentation is herein referred to as
augmentation or enhancement of the convective heat transfer
coefficient at the wall surface.
For tubular surfaces, heat transfer augmentation is achieved by
placing a helical spring-like coil made of metal or non-metal wire
or ribbon inside the tube to extend the length over which the
augmentation is desired. The outside diameter of the coil (in case
of a round tube) should be equal to or slightly less than the
inside diameter of the tube so that the wire is placed against the
wall surface or very closely adjacent to the wall. When a fluid
flows through the tube, a high level of turbulence is generated in
the boundary layer very close to the wall surface. Thus a high
degree of heat transfer augmentation takes place for a given
surface condition of the tube wall. The wire dimension; the number
of turns per unit length (spacing of the wire); fluid flow
characteristics such as velocity, density, and viscosity; and
temperature are important parameters for the heat transfer
augmentation. Therefore, the selection of parameters is important
for achieving maximum augmentation for a given system. The heat
transfer coefficient can be increased 2 to 5 times with a boundary
layer turbulator.
Where the second fluid on the outside of the tube is also moving
longitudinally with respect to the tube, an increase in heat
transfer coefficient of the same order of magnitude can be achieved
by simply slipping a boundary layer turbulator over the exterior of
the tube. Here, the inside diameter of the coil is preferably the
same as, or slightly larger than, the outside diameter of the tube
such that the wire of the coil rests on the tube wall or very
Closely adjacent to it.
According to the method of the present inventor, the heat transfer
at the inside tube wall surface is further augmented through the
provision of a compound turbulator made of a suitably dimensioned
spinner turbulator surrounded by a boundary layer turbulator,
placed inside the heat transfer tube. The wire or ribbon of the
coil comprising the boundary layer turbulator rides on the spinner
turbulator, and thus the spinner turbulator holds the boundary
layer turbulator closely adjacent to the tube wall. The centrifugal
force due to the circulatory motion of the bulk fluid drives the
boundary layer turbulence into the laminar sub-layer zone extremely
close to the wall surface and thus further increases the rate of
heat transfer using a compound turbulator. The present invention
can increase the heat transfer coefficient 3 to 8 times.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a side elevation view of a boundary layer turbulator;
FIG. 2 is a longitudinal cross-sectional view of a table with a
boundary layer turbulator installed inside;
FIG. 3 is a side elevation view of a tube with boundary layer
turbulator installed over the exterior;
FIG. 4 is a side elevation view of a compound turbulator for use in
the method of this invention, comprising a boundary layer
turbulator surrounding a spinner turbulator;
FIG. 5 is a longitudinal cross-sectional view of a tube with a
compound turbulator installed inside;
FIG. 6 is a schematic drawing showing a compound turbulator
installed inside the tube and a boundary layer turbulator on the
outside of the tube in a shell and tube heat exchanger; and
FIG. 7 is a schematic view of a compound turbulator in a fired or
unfired firetube boiler.
DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION
FIG. 1 is a side elevation view of a boundary layer turbulator 1 in
the form of a helical spring-like coil, adapted to incorporate into
a compound turbulator according to this invention. The coil can be
made of metal or nonmetal wire or ribbon. The dimension of coil is
such that when it is placed inside the tube, the wire or ribbon is
in contact with or closely adjacent to the wall. The dimension of
the wire or ribbon, and the number of turns per unit length of coil
has a strong effect on heat transfer augmentation for a given flow
and tube size.
FIG. 2 shows a boundary layer turbulator 1 installed inside a tube
3 increasing turbulence in a first fluid passing through the tube
at the inside surface of the tube wall to increase heat transfer.
The turbulator coil should extend over the length of the tube where
heat transfer augmentation is desired. For easy installation and
removal, the outside diameter of the turbulator coil is preferably
slightly smaller than the inside diameter of the tube. The first
fluid must flow through the tube in this configuration. The tube
wall can be a smooth, rough, or extended surface.
FIG. 3 shows a boundary layer turbulator installed on the exterior
surface of the tube 3 for increasing turbulence in the second fluid
passing over the exterior of the tube at the exterior surface of
the tube. For easy installation, the inside diameter of the
turbulator coil is preferably slightly larger than the outer
diameter of the tube. The turbulator coil should extend over the
length of the tube where heat transfer augmentation is desired. The
second fluid preferably flows longitudingly over the exterior of
the tube, in a direction parallel to the tube's axis. The tube
exterior can be a smooth, rough, or extended surface.
FIG. 4 shows a compound turbulator consisting of the boundary layer
turbulator 1 surrounding a spinner turbulator 2a. For easy
installation, the width of the strip forming the spinner turbulator
is preferably slightly less than inside diameter of the coil
forming the boundary layer turbulator. This compound turbulator is
far superior to a boundary layer Turbulator for heat transfer
augmentation. The compound turbulator increases turbulence
throughout the first fluid flowing through the tube, but it also
offers more resistance to flow. This increased turbulence in the
first fluid causes a thorough mixing of the first fluid, so that
the first fluid passes through the tube as a thoroughly mixed,
substantially homogenous fluid. FIG. 5 shows a compound turbulator
2 installed inside a tube 3.
FIG. 6 is a drawing of a typical shell and tube heat exchanger
which consists of a shell 4 and two tube sheets 6. A heat exchanger
tube (or tubes) 3 connects the two tube sheets as shown. Heat
transfer at the inside tube surface is augmented by inserting a
compound turbulator 2 inside the tube, and passing the first fluid
over the compound turbulator. The first fluid enters through inlet
7 in plenum 11, flows through the tube 3 over the compound
turbulator 2, into plenum 12, and then leaves the exchanger through
outlet 8. The second participating fluid enters at one end into the
shell at inlet 9 (for the counter flow situation as shown), flows
over the tube 3 (which may have an external boundary layer
turbulator thereon) exchanges heat, and finally leaves the
exchanger at outlet 10. Because of the increased turbulence in the
first fluid caused by passing the first fluid over the compound
turbulator this method effectively enhances heat transfer.
Therefore, the number of tubes, and size of the exchanger, can be
reduced without sacrificing the thermal performance of the
equipment. A multi-tube exchanger can be designed and constructed
to fulfill the requirements for a wide range of applications. The
compound turbulator 2 is useful for further augmentation at the
tube's inside surface in heat exchangers transferring heat between
(a) liquid and liquid, (b) liquid and gas, and (c) gas and gas.
FIG. 7 is a schematic drawing of a two-pass firetube fired or
unfired boiler which consists of a shell 18, a furnace tube 14
(optional for unfired boiler), and flue tubes 3. A burner for a
fired boiler is mounted at 13 or a hot gas supply for unfired
boiler is connected at 13. Thus, hot gas flows through furnace tube
14 in plenum 15, and then flows through the flue tubes 3 into
plenum 16, and finally leaves the boiler at outlet 17. The water
level inside the boiler is maintained at level 19 so that the
furnace tube and all flue tubes are always immersed in water
completely. Water is fed into the boiler at inlet 20 and steam
leaves the boiler at outlet 21. Compound heat transfer according to
the method of this invention is achieved by placing compound
turbulator 2 inside the flue tubes 3. A typical boiler has a large
number of flue tubes. Therefore, increasing heat transfer can
reduce the required number of flue tubes and overall size of the
boiler significantly, thereby reducing the cost. Also, in an
existing boiler, the use of compound turbulators improves the
thermal performance significantly and the savings involved
justifies the cost of installation of these types of
turbulators.
* * * * *